Known carburetors are classified into two types, a fixed venturi type and a variable venturi type. Variable venturi type carburetors have been mainly mounted on a specially designed car such as a sports car. Variable venturi type carburetors have the advantages of: (a) good transient response which takes place when the supply of the mixture is changed, and; (b) good continuity of operating characteristics between the idling condition and the running condition of the engine, because the variable venturi type carburetors are not divided into a slow system and a main system. Consequently, to reduce harmful contaminants in exhaust gas, reduce fuel consumption of an engine and increase the driveability of a vehicle, variable venturi type carburetors are also utilized in regular type automobiles.
As is well known in the art, a variable venturi type carburetor is provided with a suction piston movable in a direction perpendicular to an air flow, in accordance with changes in the air flow, so as to vary the area of venturi opening formed between the bottom end of the suction piston and the wall of the carburetor body facing the bottom end of the suction piston, for maintaining the speed of the air flow passing through the venturi opening substantially constant. However, in a conventional variable venturi type carburetor, a suction piston may cause a vertical oscillation due to the slow oscillation of the intake vacuum of the engine when the engine is operated at low speed. In addition, when the throttle valve disposed in the carburetor is opened suddenly (when the engine is increasingly accelerated), the suction piston may suddenly displace upward and may fluctuate widely on both sides of the predetermined position, and as a result, the air fuel ratio may become lean.
To obviate the above-mentioned problems, a conventional variable carburetor has an oil damper installed within a piston rod for guiding the suction piston. However, the oil stored in the oil damper of the variable venturi type carburetor may be lost with the passage of time during which the variable venturi type carburetor is used. If damping oil is not supplied adequately, the carburetor may cause a wide fluctuation of the suction piston or a slow oscillation of the suction piston as a variable venturi type carburetor having no oil damper disposed therein does. As a result, the operating characteristics of a vehicle, such as the acceleration ability and driveability, are degraded.
The viscosity of damping oil stored in the oil damper must be adequately adjusted. If damping oil having a high viscosity is used, the suction piston is prevented from quick movement. As a result, the air fuel ratio is enriched, harmful contaminants contained in exhaust gas are increased and fuel consumption is increased. On the other hand, if damping oil having a low viscosity is used, the damping effect of the oil damper is decreased. In addition, it should be noted that the viscosity of oil varies as ambient temperature is varied. Therefore, in summer, the problem caused by using oil having a low viscosity may occur, and in winter, the problem caused by using oil having a high viscosity may occur. As is apparent from the above discussion, an oil damper is troublesome because it requires the maintenance and adjustment of the quality and quantity of oil at a predetermined level.